1. Field of the Invention
This invention relates generally to thermodilution catheters of the type that have a heating element which applies heat to a patient's blood for the purpose of measuring a physiological condition, such as volumetric blood flow. More specifically, the invention relates to systems and methods for controlling the application of power to the heating element in order to prevent harm to the patient or damage to the catheter.
2. Description of the Prior Art
It has recently become known in the art that cardiac output can be continuously measured by utilizing a heating element on a thermodilution catheter. By applying a known thermal signal on a regular or continuous basis, cardiac output measurements can be made continuously by detecting the thermal signal downstream from the input signal and by applying sophisticated signal processing techniques. One such cardiac output measurement system is disclosed in U.S. Pat. No. 4,507,974 to Yelderman which discloses the use of a heat signal which is generated according to a pseudo-random binary sequence (PRBS). Correlation techniques are then used at the downstream position to extract the flow rate of the blood. An improved version of this system is disclosed in U.S. Pat. No. 5,146,414 to McKown, et al. The McKown et al. patent discloses a recursive or adaptive processor and uses the lagged normal model for the cardiac system in order to enhance the signal to noise ratio of the sensed downstream thermal signal.
In order for a heating element to be utilized on or in a catheter that is to be placed in a human body, certain safety requirements must be met. Most importantly, the heating element must be controlled to insure that it is operating within certain acceptable temperature ranges. Additionally, certain fail-safe mechanisms must be utilized to insure that the heating element does not get too hot. In addition to the normal temperature controls, it would be useful to be able to determine when the heating element is either subject to low flow conditions, operating in air, or otherwise under operating conditions wherein any excess heat is not being adequately dissipated.
One such prior art thermodilution heating element catheter utilizes a resistive heater having a known thermal coefficient of resistance. By monitoring the patient's blood temperature and the heating element resistance it is possible to monitor and automatically control the temperature and heat output of the heating element temperature. Certain embodiments of such a resistive heater thermodilution catheter and heater control system are disclosed in the following related U.S. Patent Applications: Ser. No. 08/049,231, to Quinn, et al., Ser. No. 08/245,727, to Yelderman et al., and Ser. No. 08/334/443, to McKown et al, (hereinafter collectively referred to as Quinn et al.) assigned to Interflo Medical, Inc., and incorporated herein by reference as though fully set forth. In alternate systems, a thermistor or thermometer is used in conjunction with the heater in order to continuously monitor the temperature of the heating element.
The catheter heating element disclosed in Quinn et al, has proven to be very safe in that it provides a means to continuously monitor and automatically control the application of power to the heating element, thereby controlling its temperature. Certain fail-safe systems have thus been incorporated into a heat-based continuous cardiac output system which utilizes the teachings of Quinn et al. An example of such a system was marketed in the VIGILANCE.RTM. cardiac output monitor marketed and sold by Baxter Healthcare Corporation, a subsidiary of Baxter International, Inc., the assignee of the present invention. The VIGILANCE.RTM. system incorporates certain fail-safe mechanisms in order to avoid the application of excessive heat to the patient's bloodstream. These fail-safe mechanisms come into play when the heating element rises above a certain predetermined temperature level.
Although the use of the Quinn et al. catheter in conjunction with the VIGILANCE.RTM. system has proven to be safe for human clinical use, there would be further utility in being able to determine, more swiftly and redundantly, when the heating element catheter is operating in very low flow conditions or in "no flow" conditions outside of the human body. Although the VIGILANCE.RTM. system provides for reducing power to the heating element when it reaches or exceeds its predetermined temperature level in order to prevent damage to the patient, it would be useful to provide a redundant control of the heating element under low flow conditions while the catheter is present in the patient's body.
It would also be useful to reduce power to the heating element while the catheter is outside the patient's body. For example, just prior to calibration of the VIGILANCE.RTM. system, power is supplied to a calibration circuit. If power is inadvertently supplied to the heating element while the catheter is under a zero flow condition outside the patient's body, it could result in thermal deformation in the catheter tubing in the region of the heating element.
It would also be useful to know, while the catheter was in the patient's body, that power being applied to the system was not going to the heating element, but was instead going to other parts of the system, such as the calibration circuit or resistor.